New evidence suggests geographic separations are less important for evolution than previously thought.
We know that natural selection drives the development of new features in living things, but what causes the walls to go up between two individuals that are both descended from the same ancestors so that they can no longer produce fertile offspring? When Darwin studied the Galapagos, the explanation was simple; the characteristics and, we now know, genetics of animals isolated on an island change. After sufficient time they become so different from those on another island that where there was one species, there is now two.
However, the situation is much less clear in other circumstances. Without frequent divergence, known as speciation, we would not have the staggering diversity we do today. But while some drivers of speciation are known, others remain controversial.
Allopatric speciation occurs because of a geographical barrier. For example, a river too wide for an animal to cross divides its territory, sending those on either side on different paths. Many generations latter the barrier may be breached, and each new species may invade the other's patch, but they have diverged enough that they cannot reunite.
The Neotropic, stretching from southern Mexico to southern Brazil, is the most biologically diverse region on Earth. A paper in Nature notes that “the sundering of populations associated with the Andean uplift” has been regarded as the primary cause of the diversification of bird species in this region. In addition to the barrier the mountains themselves represent, the formation of the Andes also changed the direction of the rivers to the east.
However, the authors argue, “An alternative model posits that rather than being directly linked to landscape change, allopatric speciation is initiated to a greater extent by dispersal events, with the principal drivers of speciation being organism-specific abilities to persist and disperse in the landscape. Landscape change is not a necessity for speciation in this model.”
A study of the genetics of 27 Neotropical bird species shows that they diverged at different times from each other, a finding hard to reconcile with the idea of mountains cutting a variety of birds off from each other at the same time. Moreover, much of the divergence seems to have taken place well after the Andes did most of their growing.
“The strongest predictors of speciation are the amount of time a lineage has persisted in the landscape and the ability of birds to move through the landscape matrix,” the authors conclude.
"The extraordinary diversity of birds in South America is usually attributed to big changes in the landscape over geological time, but our study suggests that prolonged periods of landscape stability are more important," said Robb Brumfield of Louisiana State University.
Geographic features such as mountains, rivers and the narrow Panama Isthmus are still important, but they form what the authors call “semi-permeable barriers”. Birds cross them occasionally, but not often enough for interbreeding to prevent divergence. Instead of speciation happening in a brief pulse as different bird lineages were divided by the same event, the longer a lineage survives the more likely it is divergence will occur. Birds with less mobility, such as those on the forest floor, were more likely to speciate than those in the canopy.
Ominously, it seems humans are not only wiping species out, but preventing the formation of new ones. Brumfiled says, "Our results suggest that human alterations of the landscape can effectively kill the speciation process.”